Primary central nervous system lymphomas (PCNSL) are usually diffuse large B-cell non-Hodgkin's lymphomas (NHL). Here we characterize the clinical presentation, course and outcome of patients with low-grade PCNSL. Records of 332 patients screened for inclusion in three multicentre prospective trials were reviewed. Ten patients (3%) with a median age of 59 years and a median Karnofsky performance status of 70% were identified. Seven patients had B-cell and three had T-cell lymphoma. The median growth fraction was 4%. The radiological morphology was unusual for PCNSL in eight patients. Three patients underwent complete tumour resection, combined with chemotherapy in one patient and with chemotherapy plus local radiotherapy in another. Four patients received chemotherapy and three received chemotherapy plus whole-brain irradiation, resulting in four complete remissions, two no-change situations and one progressive disease. Patients had an overall survival (OAS) of 2-58+ months with a 2-year OAS of 67%. Low-grade PCNSL may differ from classical high-grade PCNSL in its clinical features and radiological morphology. The clinical course may be variable and frequently more indolent than in classical PCNSL.